1. Field of the Invention
This invention relates to a device and method for determining in situ stresses and/or strength characteristics of a subterranean formation. More particularly, the invention relates to a device for insertion in a borehole extending into a formation, which device when pressurized exerts force radially against the borehole wall in all directions except along one plane dividing the device longitudinally.
There are many situations, such as during mining of coal or other mineral from a subterranean formation, where it is desirable to know the existing in situ stresses and/or the strength characteristics of a particular formation. Such knowledge is useful in planning the developement of the formation, and particularly as to the safety of the operation.
2. Description of the Prior Art
There are many procedures of varying degrees of usefulness which have been previously utilized in attempts to analyze the stresses and strength characteristics of subterranean formations. One method of determining the strength of a formation has been to seal a portion of a borehole with packers and then pressurize the sealed section. The formation fracture pressure can be determined in this manner, but this only gives information regarding the weakest direction. It is desirable to have information regarding stresses and strength in a plurality of directions and at a plurality of locations in order to analyze a formation by non-destructive methods. Such information is useful in techniques such as finite element analysis of force distribution within a formation.
The present invention makes possible a non-destructive "microseismic" method of formation analysis because of its capability for providing information in a plurality of directions which reflects the forces acting on the formation at a given location.